Cooling demand contributes significantly to end-use of energy globally and is a major driver of peak electricity demand. Air conditioning of buildings, for example, accounts for 15% of the primary energy used to generate electricity in the United States. Passive cooling strategies that cools without any electricity input can therefore have an impact on global energy consumption. To achieve cooling, a temperature below the ambient air temperature is reached and maintained. At night, passive cooling below ambient air temperature can be achieved using radiative cooling. Radiative cooling exposes a device to the sky to radiatively emit to outer space through a transparency window in the atmosphere between 8-13 micrometer wavelength range. Peak cooling demand, however, occurs during the daytime. Daytime radiative cooling can be difficult due to heating of the device by the sun.
For buildings (and other structures), cooling is a larger issue when the temperature is higher and when the building is exposed to direct sunlight, both of which happen during daytime. Daytime radiative cooling can therefore be significantly more useful than nighttime cooling, but is also often much more challenging due to the problem of absorbed solar radiation.